🔬 OBSERVED/INTERACTIVE STATION · CPSP FORMAT · 7 MINUTES · 15-MONTH-OLD WITH GASTROENTERITIS
📋 Observed Station – “15-month-old with vomiting, diarrhea, shock”
👧🏻 Clinical Presentation (TOACS – read by candidate / displayed):
A previously healthy 15-month-old girl presents to the emergency department with a 3-day history of persistent vomiting and non-bloody, non-bilious diarrhea. She has had little oral intake in the last 24 hours. Fever (38.6°C). On examination: tired, pale, poor response to stimuli.
Vitals: HR 140 bpm, RR 28/min, BP 95/58 mm Hg. Peripheral pulses difficult to find, capillary refill ~4 seconds, systolic ejection murmur (likely flow murmur). Liver not enlarged.
🎯 Task (observed examiner): Assess cardiovascular status, decide next intervention, estimate % dehydration, propose acute management based on Nelson Chapter 73/75.
❤️ HR 140 bpm (tachycardia)
🩸 BP 95/58 (compensated shock)
⏱️ Capillary refill 4 sec (prolonged)
🌡️ Fever 38.6°C
💧 Dehydration suspected → moderate-severe
🚨 Cardiovascular status interpretation: This child shows signs of compensated hypovolemic shock (tachycardia, difficult pulses, prolonged cap refill, normal systolic BP). The ejection murmur is likely flow-related from increased contractility/hyperdynamic state.
💡 Examiner instruction (interactive): Candidate must verbalize (1) hemodynamic status, (2) immediate intervention – fluid bolus, (3) estimate % dehydration based on clinical signs, (4) best acute indicator of hydration status, (5) complete rehydration plan (deficit + maintenance + ongoing losses). Reference Nelson Chapter 73 (osmolality, sodium, water balance) and Chapter 75 (deficit therapy).
❓ Q1 (Examiner): “Based on the scenario, describe the child’s cardiovascular status. Is she in shock?”
✅ Answer: Yes, compensated hypovolemic shock. Tachycardia (140 bpm), difficult peripheral pulses, prolonged capillary refill (4 sec), but BP is preserved (95/58) → compensation. The systolic murmur is likely a flow murmur from hyperdynamic circulation. No hepatomegaly suggests no primary cardiac failure.
❓ Q2 (Examiner): “What is the next best immediate intervention?”
✅ 20 mL/kg isotonic fluid bolus (Normal saline or Lactated Ringer) over 20 minutes, reassess after each bolus. May repeat if signs of shock persist. This restores intravascular volume and improves tissue perfusion.
❓ Q3 (Examiner): “Which is the best acute indicator of hydration status in a child?”
✅ Weight change (acute weight loss) is the gold standard. Other useful bedside signs: prolonged capillary refill, abnormal skin turgor, sunken eyes, dry mucous membranes, decreased urine output. Tachycardia is sensitive but not specific.
❓ Q4 (Examiner): “Estimate this child’s percentage of dehydration based on clinical signs.”
✅ Moderate to severe dehydration: 10–12% (infant >10% = severe). Clues: lethargy/“little response”, very delayed cap refill (4 sec), difficult pulses, tachycardia, sunken appearance. According to Nelson Table 75.1: severe dehydration (>10% in infant) includes poor perfusion, weak pulses, decreased BP (BP here borderline but still compensated).
❓ Q5 (Examiner): “What type of dehydration is most likely (isotonic, hyponatremic, hypernatremic) and why?”
✅ Most likely isotonic dehydration (hyponatremic possible). Gastroenteritis with vomiting + diarrhea leads to loss of hypotonic or isotonic fluid. If child replaced with plain water or low-salt fluids, hyponatremia can occur. Initial labs would guide: Na typically 130–145 mEq/L. Treatment with isotonic saline is safe for any variant initially.
❓ Q6 (Examiner): “How do you calculate 24-hour fluid requirement for this child (weight approx 10 kg)?”
✅ Maintenance (Holliday-Segar): 100 mL/kg/24h for 10 kg = 1000 mL. Deficit (10% dehydration): 10 kg × 10% = 1000 mL deficit. Total 24h = maintenance + deficit = 2000 mL. Subtract fluid bolus (20 mL/kg = 200 mL) given initially → remaining 1800 mL over 24h plus ongoing losses.
❓ Q7 (Examiner): “What IV fluid composition is recommended for maintenance + repletion after initial bolus?”
✅ Isotonic fluid with 5% dextrose and 20 mEq/L KCl (e.g., D5 0.9% NS + 20 KCl) after the patient voids and has normal renal function. Recent guidelines recommend isotonic maintenance fluids to prevent hyponatremia. Avoid hypotonic solutions in hospitalized children with non-osmotic ADH stimuli.
❓ Q8 (Examiner): “When do you add potassium to IV fluids in a dehydrated child?”
✅ Only after adequate urine output is established and serum potassium is known (not elevated). Typically after first void, but in severe dehydration withhold until renal function confirmed. Potassium replacement is critical because diarrheal losses cause total body K+ depletion.
❓ Q9 (Examiner): “What is the best way to monitor response to fluid therapy?”
❓ Q10 (Examiner): “What acid-base disorder is common in diarrheal dehydration and why?”
✅ Hyperchloremic metabolic acidosis (normal anion gap) due to loss of bicarbonate-rich stool. May also have lactic acidosis if hypoperfusion present (increased anion gap). See Nelson Chapter 73.7 – diarrhea causes loss of HCO3-, leading to metabolic acidosis.
❓ Q11 (Examiner): “If this child had hypernatremic dehydration (Na >150), what special precaution is needed?”
✅ Correct slowly: decrease serum Na by no more than 10–12 mEq/L/24h to avoid cerebral edema. Use fluid with higher sodium concentration (e.g., D5 0.9% NS or D5 0.45% NS) and monitor Na frequently. Rapid correction risks osmotic demyelination or cerebral edema due to idiogenic osmoles.
❓ Q12 (Examiner): “When do you consider using vasopressors in pediatric hypovolemic shock?”
✅ If fluid-refractory shock (no improvement after 60 mL/kg or more of isotonic boluses) and signs of poor perfusion persist, especially if warm or cold shock with low BP. In such cases, after ensuring adequate volume, start inotropic support (dopamine, epinephrine). But hypovolemic shock from gastroenteritis rarely requires vasopressors if volume repleted adequately.
🩺 Examiner tip: Know the formula for maintenance (4-2-1 rule), deficit calculation, and the recent shift to isotonic maintenance fluids (avoid hypotonic solutions) to prevent iatrogenic hyponatremia (Nelson Chapter 74).
💧 Dehydration Management · Step-by-Step Plan (Nelson Ch 75)
📝 Estimate & action plan for this 15-month-old (estimated weight 10 kg):
1. Resuscitation phase (shock)
- Immediate IV/IO access → 20 mL/kg normal saline bolus over 15–20 min. Reassess. May repeat up to 60 mL/kg.
- After stabilization: maintenance + deficit.
2. Deficit calculation (assuming 10% dehydration)
- Deficit = 10 kg × 10% = 1000 mL.
- Maintenance (Holliday-Segar) = 1000 mL/24h.
- Total 24h = 2000 mL.
- Subtract initial bolus (200 mL) → 1800 mL over remaining 24h.
3. Fluid composition
- D5 0.9% NS + 20 mEq KCl/L (after void, normal K+).
- Ongoing diarrhea loss replacement: replace mL/mL with similar electrolyte solution (e.g., D5 1/2NS + 30 mEq NaHCO3 + 20 KCl) if large losses.
📊 TOACS - Oral rehydration alternative: For mild-moderate dehydration (without shock), ORS (75 mEq/L Na, 75 mmol/L glucose) can be used, but this child has severe dehydration with shock → IV route mandatory.
🩸 Signs of severe dehydration (>10% infant) Lethargy, weak/absent pulses, hypotension, cool mottled skin, no urine output, sunken eyes, very delayed cap refill (>3 sec).
📐 Maintenance fluid calculator 10 kg: 1000 mL/day; 4 mL/kg/h. After initial bolus, run at ~75 mL/h for repletion + maintenance (adjust based on ongoing losses).
⚖️ Weight change gold standard Daily weight best indicator. 1 kg loss = 1 L fluid deficit.
🧪 Electrolyte pitfalls Hypokalemia may worsen after rehydration (K shifts intracellular). Monitor, replace after urine output.
🧠 Common TOACS question: “Child vomits during oral rehydration” → Switch to IV fluids if unable to retain; NG tube can be used for rehydration if IV access difficult.
📘 Nelson Textbook Highlights · Fluid & Electrolyte Disorders
💧 Total body water Infant 75% → 60% by 1 year. ECF larger in infants, vulnerable to dehydration.
🧂 Sodium and osmolality Effective osmolality = 2×Na + glucose/18. ADH regulates water balance; volume depletion overrides osmolality.
⚡ Hypernatremia treatment Correct slowly (≤10 mEq/L/24h). Avoid rapid drop → cerebral edema. Use isotonic fluids initially then adjust free water.
🔁 Potassium Intracellular cation. Hypokalemia in diarrhea due to fecal losses. Replace after urine output established.
🩸 Metabolic acidosis in diarrhea Loss of bicarbonate → hyperchloremic metabolic acidosis. Calculate anion gap to differentiate from lactic acidosis.
💉 Deficit therapy (Ch 75) Estimate % dehydration, give isotonic bolus, then 24h maintenance + deficit. For hypernatremia, extend repletion over 48h.
📈 Monitoring Serial weight, UOP, electrolytes (Na, K, Cl, HCO3), BUN/Cr. Urine specific gravity and osmolality helpful.
📚 Nelson 22nd Edition Chapter 73.1–73.3 & 75: “In hypovolemic shock, the first priority is restoration of intravascular volume with isotonic crystalloid (20 mL/kg). Children with severe dehydration often have a metabolic acidosis that corrects with volume repletion without needing exogenous bicarbonate.”
🔗 Additional Resource: Visit paeds.online for FCPS Paeds TOACS scenarios, fluid calculators, and electrolyte practice.